1. Field of the Invention
The present invention relates to an image sensing apparatus and a method for detecting defective pixels of an image sensor.
2. Description of the Related Art
Pixels of a photoelectric transducer such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) sensor include defective pixels disabled to generate normal output signals due to crystal defects or dusts. Such defective pixels are mainly classified into the following two types in terms of time characteristics.
One is a regular defective pixel that cannot output any normal output signal at any time (abnormal output, hereinafter). The other is a flicker defective pixel that irregularly outputs a normal output signal and an abnormal output signal.
In the CMOS sensor, output characteristics are different between the regular defective pixel and the flicker defective pixel. In the regular defective pixel, white defects caused by crystal defects of a light receiving portion are predominant except for sensitivity-dependent defective pixels such as dusts or uneven apertures. The white defects are accompanied by an increase of dark signals, and hence an abnormal output level depends on a temperature and storage time.
In the case of the flicker defective pixel, places where crystal defects occur are different from those of the regular defective pixel, and hence an abnormal output level has almost no dependence on a temperature or storage time.
Referring to FIG. 6, a place where a crystal defect of a flicker defective pixel occurs will be described.
FIG. 6 illustrates an example of a general circuit of a pixel portion of the CMOS sensor. This circuit includes a photodiode (PD) 601 (a light receiving portion), a reset MOS 602 for resetting accumulated charges, a floating diffusion (FD) 603 for detecting charges, and a pixel source follower MOS 604. Many of regular defective pixels are caused by crystal defects of the PD 601.
On the other hand, flicker defective pixels are generated in the pixel source follower MOS 604, probably due to repetition of capturing and discharging electrons in interface order of a MOS transistor.
The abnormal output level of the flicker defective pixel has almost no temperature or storage dependence, and hence detection thereof has been difficult by the same defective pixel detection method as that for the regular defective pixel.
As a solution to the above situation, Japanese Patent Application Laid-Open No. 2005-341244 discusses a method for executing image-capturing of a plurality of frames, and specifying a defective pixel based on the number of times the output thereof has exceeded a predetermined threshold value.
The use of the method for executing image-capturing of the plurality of frames and specifying the defective pixel based on the number of times the output thereof has exceeded the predetermined threshold value enables detection of a flicker defective pixel.
However, depending on image-capturing conditions during defective pixel detection, abnormal output levels vary between the regular defective pixel and the flicker defective pixel. Thus, accurate detection of both defective pixels is difficult.
For example, the regular defective pixel depends on temperature and storage time, and hence its abnormal output level becomes higher under image-capturing conditions of high temperature and long-second storage. The regular defective pixel has its abnormal output level changed depending on a temperature and storage time. Thus, to accurately detect the regular defective pixel, a threshold value for an output for determining whether a pixel is a defective pixel has to be set higher as the temperature and the storage time increases.
On the other hand, the flicker defective pixel has no temperature or storage time dependence, and hence its abnormal output level does not become higher even under image-capturing conditions of high temperature and long-second storage.
Thus, as temperature becomes higher and storage time (second) becomes longer as conditions during defective pixel detection, an abnormal detection level of the regular defective pixel becomes higher as compared with that of the flicker defective pixel.
If a threshold value for an output for determining whether a pixel is a defective pixel is set by using the abnormal detection level of the regular defective pixel as a reference, a flicker defective pixel that is not dependent on temperature or storage time is difficult to be detected. As a result, even if defective pixel correction is carried out, an image is formed where a defective pixel that is easily detected at a low temperature and at short-second storage stands out.
For determination, when a lower temperature and shorter-second storage are set as conditions for defective pixel detection, the abnormality detection level of the regular defective pixel becomes lower as compared with that of the flicker defective pixel.
Thus, if a threshold value for an output for determining whether a pixel is a defective pixel is set by using the abnormal detection level of the flicker defective pixel as a reference, a regular defective pixel dependent on temperature and storage time is difficult to be detected. As a result, even if defective pixel correction is carried out, an image is formed where a defective pixel that is easily detected at high temperature and long-second storage stands out.